Regulation Of Host Populations And Relative Abundance In Communities

Parasites create a diversity of links in food webs that at first sight may appear atypical, but they are not unusual in nature—more than 75% of links in natural food webs probably involve parasites (Lafferty et al., 2006b). Because many parasites use multiple competing hosts on the same trophic level, their population dynamics may be modeled by sets of coupled differential equations that take the general form dst/dt = b(S + It)-dS - j¿

j=i dil/dt = j ¿Ij-d)ii j=i where we assume that each host species i has species-specific birth and death rates (b and d) and experiences transmission of the pathogen at a rate P„ from infected individuals of species j. Infection converts each susceptible host, S, into an infectious individual, I, that experiences an increased pathogen-induced mortality rate, a. When compared with single-species infectious disease models, the presence of interspecific transmission is usually strongly stabilizing for a wide range of interspecies transmission rates that are less than the rates of within-species transmission (Dobson, 2004). However, when rates of interspecific transmission approach rates of within-species transmission, the pathogen acts as a powerful mechanism of indirect competition [as a shared natural enemy (Holt and Lawton, 1994)] that can drive some host species extinct.

We can examine the potential consequences of this for more complex systems by recasting the differential equation models within the matrix framework that describes the initial trajectory of a perturbation to the whole food web. Thus, each element of the matrix represents a pairwise interaction between each pair of species in the food web (Pimm, 1982; Pascual and Dunne, 2005). If we retain our classification of each host as susceptible and infected, then the parasite in effect enters the food web as two species. Both have the phenotype of the host (although the feeding preferences might change after infection). However, the infected hosts now effectively have the genotype of the pathogen, and transmission acts as a birth process converting susceptible hosts into infected individuals that can also be considered as ''shared natural enemies'' of uninfected hosts of all susceptible species. We can briefly examine a submatrix of food web interactions for specialist and generalized pathogens within a food web.

Specialist parasites and competing host species A Ia B Ib

Pathogens shared between competing host species

In these two matrices of species interaction, host species A and B compete with each other for resources such as food or space, and each host species has a pathogen associated with it (thus infected hosts of species A are characterized as ''species'' Ia). In the case of specialist parasites (upper matrix), infected hosts of species A cannot infect species B; the complementary case operates for the lower matrix, where both species of pathogen infect both species of host. The main consequence of host species sharing nonspecific parasite species is that several elements of the interactions matrix have to be converted (across the main diagonal) from ''zeros'' into ''plus-minus'' consumer-resource relationships. If we are concerned with the stability properties of the web, then May (1973) has shown that the dominant eigenvalues of this matrix have to be negative if there is to be any hope of web stability. In May's initial formulation, increased species diversity and hence increased connectance should reduce the probability that the web is stable. However, although the net effect of shared pathogens is to increase the connectance of the food web, this occurs in a subtle and important way. Namely, the conversion of specific pathogens to generalized pathogens greatly increases the proportion

of ''across-diagonal'' plus-minus links in the web. Because the product of their interaction is always negative, adding more summed negative terms increases the chances that this eigenvalue will be negative (Allesina and Pascual, 2008). More specifically, adding shared pathogens to the food web significantly increases the proportion of negative cross-product terms relative to positive product terms produced by competition (where negative times negative = positive!). This effect generalizes: As we increase the species diversity of the web, destabilizing competitive interactions will increase at a maximum rate of (n2 - n)/2, whereas potentially stabilizing shared pathogen interactions increase at the significantly faster maximum rate of n2.

Similar effects arise when we consider parasites with complex multiple host life cycles. These infectious agents confound traditional concepts of food-web structure because they feed on several different trophic levels within different host species during the course of their life cycle. They also act as food resources to species on different trophic levels as they pass through their free-living stages. Usually, <1% of the energy-rich, free-living infective stages of a parasite ever manage to infect a host; the other 99% are eaten by planktivorous species. Parasites with this type of life cycle can again be incorporated into food-web models. Initial results with matrix models of the form described above suggest that such parasites will also have key stabilizing effects on the structure of food webs because they also add pairwise sequences of ''plus-minus'' resource-consumer interactions at every stage of their life cycle, and these will consistently increase the probability that the dominant eigenvalues of the linearized system will be negative. Furthermore, generalist parasites with complex multihost life cycles also introduce long circular loops of relatively weak links into the web; theoretical analysis by Neutel et al. (2002) suggests that these may also be important in imparting stability to food webs.

Thus, generalist parasites and those with complex life cycles potentially play important roles in regulating the relative abundance of their free-living host species. Whereas generalist species with direct transmission are likely to be buffered from extinction by the rescue effect of at least one host remaining abundant, parasites with complex life cycles will depend highly on the host species in the life cycle to which they are most specifically adapted. The trematode and acanthocephalan species that are recorded as adult worms from scores of vertebrate host species often depend entirely on a single species of mollusk or amphipod that serves as their intermediate host. Thus, snails or other invertebrates that invade natural ecosystems and replace crucial host species within the complex life cycles of parasites may lead to losses of parasite diversity that cascade throughout the food web.

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